I have a client who is making a decision on if they will go with SKUID Platform for their Portal use case or go with Salesforce Communities for that and the timing is important for me to be able to verify the use case with the trial edition.

I'm going to keep trying and see if I can get it to work and will update this post if I do. If you manage to get it working please do the same.

Andrew / Joseph, what OAuth Grant Type did you use when configuring your Auth Provider? Could you post some details / screenshots of your Auth Provider setup? That document you referred to in the original post describes how to use either the "Authorization Code" or "Resource Owner Password Credentials" grant types --- which are you using?

Either way, there are some bugs which affect creation of Auth Providers which will hopefully get rolled out to production later this week --- these bugs might be affecting you here, but I would need more details as to your Auth Provider setup in order to verify.